A lanthanum cerium terbium phosphate phosphor is disclosed in U.S. Pat. No. 4,423,349 to Nakajima et al.
Nakajima et al. disclose, however, that when a small amount of gadolinium is added to the composition, the brightness of the phosphor is less than that of the phosphor without the gadolinium. See Table 5, particularly column 6, line 58 (second entry in Table) and column 7, line 5.